The study of the relationships between annual rainfall and range production in the Mediterranean Basin and the Sahelian Sudanian tropical zones of Africa shows a close correlation between average range production and average rainfall over large geographic areas. For a given amount of rain, net production is higher in the Mediterranean than in the Sahel. The regressions developed show that, on the average, each millimeter of rainfall produces 2 kg/ha of consumable dry matter, or 0.66 Scandinavian Feed Units (FU), in the Mediterranean, whereas in the Sahelian and Sudanian zones these figures drop to 1 kg/ha and 0.40 FU. Pasture production depends on various factors such as climate, nature of soil, botanical composition and vegetation structure, and type and intensity of management, e.g., grazing patterns and stocking rates, fire, and wildlife (including insects and rodents). Since all these factors or group factors can limit yields, one should be able to integrate them all in a single mathematical model. Such a model would make it possible on the one hand to assess potential yield of grazing lands, and on the other to determine, through simulation, the effects on these yields of any modification in one or several factors. Some attempts towards this aim have already been made for the Sahel (Seifert and Kamrany 1974; Picardi 1975). Data are now being collected in order to build such models, and the programming of simulation models is being studied as well. In order to form some idea of the importance of climatic factors on the production of natural pastures, an attempt was made to correlate this production with average annual rainfall in two large areas: the Mediterranean Basin and the Sahelian and Sudanian zones of Africa. The average annual rainfall was selected because of its apparent importance where forage production is concerned. Many authors have attempted to relate pasture production to average rainfall, e.g.,

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Owing to increasing scientific and agricultural interest in the disease-resistant (trypanotolerant), indigenous cattle breeds of West and Central Africa, there is a need for a rational genetically based description of populations in the region. The greatest threat to the invaluable genetic resource represented by these animals is that of extensive genetic introgression of distantly related zebu cattle from northern populations which do not share their inherited tolerances. Southern blotting with a chromosome Y-specific probe, btDYZ-1 (locus DYZ1) is shown to be a sensitive assay to detect such introgression. Evidence of historical crossbreeding is reported in two important N'Dama populations previously classed as purely taurine.

Summary Owing to increasing scientific and agricultural interest in the disease‐resistant (trypanotolerant), indigenous cattle breeds of West and Central Africa, there is a need for a rational genetically based description of populations in the region. The greatest threat to the invaluable genetic resource represented by these animals is that of extensive genetic introgression of distantly related zebu cattle from northern populations which do not share their inherited tolerances. Southern blotting with a chromosome Y‐specific probe, btDYZ‐1 (locus DYZ1) is shown to be a sensitive assay to detect such introgression. Evidence of historical crossbreeding is reported in two important N'Dama populations previously classed as purely taurine.

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